Cortana was created specifically for Operation: RED FLAG, a SPARTAN-II mission to capture a Covenant ship and kidnap a high-ranking member of the Covenant hierarchy. She was to serve as the Spartans' codebreaker, hacker and intelligence lead in the mission. As a result, an exceptionally intelligent human brain was needed to create the required AI construct. Dr. Catherine Halsey ultimately decided that she, herself, would be the best possible brain donor for the operation. Under normal circumstances, the brain donor for an AI construct must be deceased, as the organ itself is destroyed in the Cognitive Impression Modeling process. Halsey's ingenious (and highly illegal) solution was to have her brain flash cloned for the procedure. Twenty brains were tank-grown in clone bodies with enhanced neural physiology but lacking most normal bodily functions, leaving them severely misshapen. Halsey then extracted the brains and incinerated the bodily shells they were grown in. Most of the cloned brains had imperfections and had to be discarded; only one specimen was sufficiently developed for AI creation. This brain, designated as H-1, was used to create Cortana. Halsey placed the three other remaining specimens, H-2, H-3 and H-4, into cryogenic suspension for potential future use.[2]

Cortana's first words were the Italian phrase, "Quando il gioco è finito, il re e il pedone vanno nella stessa scatola ('When the game is over, the king and the pawn go in the same box')." This phrase was spoken by Halsey's mother after their first game of chess.[2] The newborn AI chose the name "Cortana" herself, and although Halsey did not tell Cortana that she had planted a standard viral termination code in her software, Halsey speculated that Cortana had already isolated and bypassed the code without her knowing.[2] Cortana spent the first three years of her life residing in the computers of CASTLE Base, often accompanying Dr. Halsey in her work. Halsey enjoyed Cortana's presence, both due to her useful skill and her personality. While smart AI are not carbon copies of their 'parents', Cortana chose a young representation of Halsey to act as her physical avatar, while also retaining some of Halsey's mannerisms. In Fall 2552, in preparation for RED FLAG, Cortana was asked by Dr. Halsey to pick a soldier that would act as her "carrier". Cortana chose John-117 for their neural compatibility, as well as his uncanny luck in battle. In her personal thoughts, Cortana considered him "attractive in a primitive animal sort of way".[9]

"They let me pick. Did I ever tell you that? Choose whichever Spartan I wanted. You know me. I did my research. Watched as you became the soldier we needed you to be. Like the others, you were strong and swift and brave. A natural leader. But you had something they didn't. Something no one saw, but me. Can you guess...? Luck."

Cortana in a holotank, analyzing the Forerunner artifact under SWORD Base

On August 3, 2552,[2] Dr. Halsey separated Cortana into two "fragments". One of these fragments, which contained all of her higher functions and personality, was stationed on board the UNSC Pillar of Autumn, while the other was tasked with mining data from the Forerunner artifact beneath SWORD Base. As the translation began to progress, Cortana's decryption of the Babd Catha artifact began to reveal what Halsey's fellow xenoarchaeologist, Laszlo Sorvad, had reported of the excavation, a "latchkey discovery", something that could change the tide of the Human-Covenant War.[11] Deciding to ensure the data's delivery to Spartans, Dr. Halsey chose to recall NOBLE Team (who, unlike the SPARTAN-IIs assembled for RED FLAG, were fully aware of the Fall of Reach), and tasked them with delivering Cortana's fragment carrying Sorvad's data to the Pillar of Autumn. By August 14[note 1] Cortana and Halsey discussed which of NOBLE's Spartans she would trust to escort her, eventually choosing SPARTAN-B312, Noble Six, noting the Spartan's "hyper-lethality", which she compared to that of John-117.[12] However, contacting NOBLE Team proved difficult, as the team had been separated following the aftermath of Operation: UPPER CUT and would not be recovered by the UNSC until August 26.[13]

Meanwhile, on August 29, Cortana's primary fragment was inserted into John-117's MJOLNIR Mark V armor for the first time, and the two then took part in a test to determine the abilities of their bond as an AI-human team.[14] The test was devised by ColonelJames Ackerson, a bitter rival of Dr. Halsey's who held contempt for the SPARTAN-II program, and by extension John-117. Ackerson incorporated armed Orbital Drop Shock Troopers, Lotus anti-tank mines, automated turrets in a razor wire field, and even a SkyHawk jumpjet into the test in order to have John killed. Despite the test being rigged, John was determined to finish without breaking the rules. With Cortana's assistance providing battlefield intelligence and boosting his reaction time to its limits, John managed to traverse the course relatively unharmed.[15]

After the test, Cortana was installed into the UNSC Pillar of Autumn as the ship's primary AI, with another AI, Wellsley, as backup. While aboard, she exacted revenge upon Colonel Ackerson by hacking into his system and blackmailing him, hoping that this would send him back to the front lines.[16] She also hacked into the SPARTAN-II highly classified personal files, and read over John's file. Cortana was shocked and divided over the dubious morality of the SPARTAN-II project, and secretly vowed to protect John to the best of her ability.[17] On August 30, as the Autumn was making final preparations for the mission into Covenant space, Captain Jacob Keyes was informed that a vast Covenant fleet had dropped out of slipspace and were moving towards Reach. The Autumn was forced to abandon its original mission and to assist in defending Reach.[18]

—Dr. Catherine Halsey, agreeing on Cortana's choice of Noble Six as her carrier

On August 29, 2552, NOBLE Team arrived at Sword Base under pretense orders of a "torch-and-burn" operation to destroy SWORD Base, being informed only at the last minute that their real mission was to deliver an armored matrix containing Cortana's fragment and the artifact's data to the Pillar of Autumn, which had docked at the Aszod ship breaking yards.[19] By this time, the decryption had been completed and responsibility of the Package was turned over to Noble Six. Although nearly every member of NOBLE tasked with the delivery was killed in the process, Cortana was successfully brought to the ship, with Captain Keyes collecting the Package personally. SPARTAN-B312 sacrificed refuge on the Autumn to cover the ship's escape and Cortana's two fragments were reunited once the Autumn took off.[20]

"You have no idea how this ring works, do you? Why the Forerunners built it? Halo doesn't kill Flood, it kills their food. Humans. Covenant. Whatever; we're all equally edible. The only way to stop the Flood is to starve them to death. And that's exactly what Halo is designed to do: wipe the galaxy clean of all sentient life."

—Cortana reveals the true purpose of Halo to John-117

Cortana and Keyes discuss the situation as John watches

On September 19, the Autumn emerged from slipspace in the Soell system, in orbit around the gas giant Threshold. It was there that the crew discovered Installation 04 in orbit around the gas giant. Cortana could only speculate as to what exactly the ring was, only able to deduce that it was artificially created.[25] The AI then informed Captain Keyes that fifteen Covenant ships had pursued the Autumn from Reach, and were moving to engage with boarding parties. Cortana assumed control of the Autumn and used its weapons to destroy four of the fifteen attacking Covenant ships. By the time John-117 had thawed from cryo-sleep and reached the bridge, the cruiser was severely damaged, and Keyes was forced to initiate the Cole Protocol. He ordered Cortana to upload herself into John's armor to protect the vital information she carried. John fought his way through the Autumn's service corridors with Cortana's guidance, and the two escaped the ship in a lifeboat. The lifeboat entered the ring's atmosphere as the Autumn was shot down.[26]

The lifeboat made a crash-landing on the surface of the ring, leaving John and Cortana as the only survivors. The two evaded Covenant patrols and rendezvoused with a group of Marines fending off a Covenant attack. Pelican Echo 419 dropped off a Warthog, and the pair set out across the surface of the ring to rescue any survivors in the immediate area. At the same time, Cortana hacked into the Covenant Battle Network, and discovered Keyes' location - being held prisoner on-board the Truth and Reconciliation.[27] The two were transported to the UNSC's base of operations on the ring, Alpha Base, and along with ODSTs of the 22nd Tactical, devised a plan to rescue Keyes. John, Cortana and several Marines were dropped off near the Truth and Reconciliation that night and boarded the ship via its gravity lift. Cortana led John and his team through the interior of the alien vessel, opening doors and marking Navigation Points. The group eventually located Keyes and several other captured crewmen in the ship's brig. Keyes had learned from the Covenant that the ring was known as Halo, and was possibly a weapon of mass destruction. The group escaped the ship, and soon learned that Halo's control room could be found using the Cartographer, a map room.[28]

The next day, on September 20, the Chief, Cortana and two squads of Marines set out for the island on which the Cartographer was located. After fighting through significant Covenant resistance, the two reached the Cartographer, and found that Halo's control room was located below the surface. As the two made their way back towards the surface, Captain Keyes informed them that he was going to investigate a supposed "weapons cache" with several other Marines. John and Cortana were retrieved by Echo 419, which descended into the interior of Halo towards the control room.[29] The Pelican continued to descend as far as it could before dropping off John and Cortana on a platform. There they traversed through a network Forerunner structures within a large underground chasm. After many firefights, the Chief and Cortana reached Halo's control room, where the AI was uploaded to Halo's core systems. Cortana was overwhelmed by the amount of data contained within Halo's systems, but discovered that the "weapons cache" Keyes and his Marines located was in fact a Flood containment facility. John left to evacuate the trapped soldiers immediately.[30]

Cortana in Halo's computer systems clashing with 343 Guilty Spark, with the Chief caught between them

After the Chief encountered the parasitic Flood for the first time, he was convinced by the Forerunnermonitor343 Guilty Spark to activate the Halo. During this time, Cortana absorbed as much data as she could from Halo's control systems. When John returned to the control room accompanied by the Monitor on September 21, he had recovered Halo's activation index, intent on using it to eliminate the Flood. John inserted the Index into the control systems, but was stopped by Cortana before it could trigger. Cortana absorbed all data from the Index, and explained to John the true purpose of Halo; to wipe the galaxy clean of sentient life as a final resort against the Flood. Guilty Spark confirmed this and summoned his Sentinels, demanding that Cortana be handed over. When John refused, Spark left his Sentinels to eliminate them. The two escaped the control room as the Flood began to run rampant across the ring, infecting human and Covenant alike. Cortana suggested that they should overload Halo's three phase pulse generators to slow the Monitor's activities. While the generators were disabled by John, Cortana discovered that the Pillar of Autumn was still structurally intact after its crash. She deduced that overloading its fusion reactors would have enough force to destroy Halo, however Captain Keyes' CNI transponders would be needed to do so. Cortana tapped into the Monitor's teleportation network and transported both herself and John to Keyes' last known position - the Truth and Reconciliation.[31]

By the time John and Cortana arrived at the Truth and Reconciliation the vessel was under attack by the Flood, and was severely damaged. Immediately upon arrival, the pair was contacted by Keyes, who weakly ordered them to leave him. After intense fighting within the ship, Cortana observed that the Flood gathered bodies of the dead for use as biomass. As Covenant forces made an attempt to retake the ship the pair was again contacted by Keyes, who repeated his order to leave him behind. Cortana was desperate to rescue Keyes, stating that the Captain was delirious from pain. Finding the source of Keyes' transmissions on the ship's bridge, the two were met with a gruesome sight - Keyes had been infected and assimilated into a Proto-Gravemind. In order to complete their objective and spare the Captain needless pain, John retrieved Keyes' CNI transponders, killing him in the process. The pair escaped the ship in a Banshee, and headed towards the crashed Pillar of Autumn.[32]

John and Cortana on the bridge of the wrecked Pillar of Autumn

On the morning of September 22, John crashed the Banshee into the side of the Autumn, much to Cortana's irritation, and the two boarded the ship and fought through Flood and Covenant to the bridge. The Chief uploaded Cortana into the ship's systems, and she began a countdown timer to overload the ships reactors. However, the countdown was halted by none other than 343 Guilty Spark, who was on board the ship, and absorbing much of its data. Spark sent his Sentinels to capture Cortana and retrieve the Index. John suggested that they overload the reactors manually with grenades and rockets, to which Cortana reluctantly agreed. The two moved back through the ship, and reached Engineering, where they overloaded the Autumn's reactors. Cortana radioed Echo 419 to extract at an external access junction on the ship. As the Autumn began to explode and fall apart, John entered a Warthog and drove down a service corridor than ran the length of the war-torn ship. However, upon reaching their extraction point, Echo 419 was shot down. In an act of desperation, Cortana discovered a GA-TL1 Longsword still docked in Launch Bay 7. With little time left, the two entered the Longsword and sped away from Halo as it was ripped apart by the exploding Autumn. Upon scanning, Cortana found nothing but "dust and echoes", thinking them the only survivors of Halo, concluding that it was finished. John however, stated that they were just getting started.[33]

Cortana: "We did what we had to do; an entire Covenant Armada obliterated, and the Flood...we had no choice. Halo...it's finished."

John-117: "No...I think we're just getting started."

—Conversation between Cortana and John-117 after the conclusion of the Battle of Installation 04[33]

Cortana and the Master Chief, seemingly the only survivors of the Battle of Installation 04, quickly discovered a small number of other UNSC personnel in a Pelican dropship.[34] Knowing that the Pelican wasn't capable of traveling long distances, the Master Chief, along with Cortana and the Pelican's four-man crew, fought their way onto the Covenant DDS-class carrierAscendant Justice.[35] Shortly afterward, Cortana took control of the ship, and returned the survivors to Reach, where the group helped to rescue survivors of the Raid of Reach, including her "mother", Dr. Halsey.[36] She managed to improve on the ship's weapons and slipspace capabilities, jumping below Threshold's atmosphere, a technique never accomplished before by the UNSC.[37] In an impressive display of cunning, Cortana quickly destroyed a Covenant AI, as well as a half dozen Covenant warships with Ascendant Justice's newly refocused plasma projectors.[38] She also copied two of the construct's functions: the ability to translate Covenant speech, and the ability to create imperfect clones of herself.[39] These capabilities, along with her exclusive knowledge about the Halos, made her invaluable to the UNSC war effort.

The sheer amount of information recovered from Halo cut into Cortana's processing power, a burden which weakened her greatly. During her time on Ascendant Justice, Cortana exhibited uncharacteristic behavior including anger, distraction, and self-doubt - an indicator that she was nearing rampancy.[40] Dr. Halsey erased some data from her memory to give her more thinking space, including data regarding Sergeant Johnson's supposed immunity to the Flood as a result of Boren's Syndrome, which was in fact a cover to hide Johnson's status as a former ORION supersoldier.[41]

The Ascendant Justice was effectively fused with the UNSC Gettysburg. It was at this point that Cortana learned of a planned Covenant assault on Earth. The Master Chief, along with SPARTAN Blue Team and a patchwork software copy of Cortana, would later execute Operation: FIRST STRIKE.[42]

Unfortunately, the Covenant AI that Cortana had decompiled had managed to, prior to its capture, send a message over battlenet, warning the Covenant of its capture and sending the necessary calculations for a sub-atmospheric slipspace jump. This information was later used by the Prophet of Regret to enter slipspace right above New Mombasa during the Battle of Mombasa.[43]

The Master Chief and Cortana would later return to Earth, at which point Cortana handed all of the sensitive data in her possession over to ONI. However, Cortana retained the Index and apparently still possessed some data on the Halo network, including critical information on the Ark.

Upon exiting slipspace, it was discovered that the Solemn Penance had led the In Amber Clad to another ringworld, Delta Halo. Cortana gave Miranda Keyes access to all information recovered from Halo ring, and provided vital intelligence to the Chief, UNSC Marines and the ODSTs deployed to the surface of the ring.[45]

After the Master Chief assassinated the Prophet of Regret on Delta Halo, he, Cortana and ArbiterThel 'Vadamee were captured by the Flood central intelligence - the Gravemind. However, it did not harm them, instead choosing to use the two as pawns to stop the Prophet of Truth from firing the Halo. The Gravemind sent the Master Chief and Cortana to the Covenant capital High Charity, where Cortana infiltrated the city's computer systems, while the Master Chief pursued the Prophet of Truth.[46] The construct promised to detonate the crashed In Amber Clad's reactors to destroy the city if the Flood proved unstoppable, or if the Halo ring was activated. However, doing so meant she would be forced to remain in the High Charity computer systems to ensure detonation. As Truth boarded the Forerunner Dreadnought in the heart of the city to reach the Ark, Cortana fought with the ancient Forerunner AI Mendicant Bias to delay the ship's launch.[47] The firing of Halo was averted by Miranda Keyes, the Arbiter, and Johnson, while John successfully boarded the Dreadnought and escaped High Charity. Cortana was left alone with the vast Gravemind to battle for her survival. She had only John's promise that he would return soon to free her.[48]

"Chief! High Charity, the Covenant's Holy City is on its way to Earth. With an army of Flood."

—Cortana's recording for the Chief.

Forced to interface with the Gravemind and under constant mental torture, Cortana nonetheless managed to send a message to the Master Chief on Earth, stashing it on an Earth-bound Flood-infected ship that crash-landed in Voi.[49] She accomplished this by creating a message and using the Gravemind's link to the ship in order to transmit it. The Gravemind permitted this in order to lure the Master Chief to destruction, but was unaware of what the message said. Cortana claimed that the Gravemind was approaching Earth in the infested High Charity, and that it was as yet unaware of the Portal and where it led to. Through the portal, there was a solution to stop the Flood once and for all. Though Lord Hood doubted her message, fearing that Cortana could have been corrupted or driven rampant, the Chief's trust in the AI never wavered. A joint UNSC-Covenant separatist task force then entered the Portal, heading to Truth and to Cortana's solution.[50] Lord Hood's instincts were correct: the Gravemind was in fact aware of the Portal and where it led (it had learned this after assimilating the Prophet of Regret) - and had deliberately allowed Cortana to send a message in order to lure all of its enemies into one place. Then the millions of Flood forms stored within High Charity would assimilate them all. Cortana, however, managed to hide the existence of Installation 04B and the index she possessed from the Gravemind.[51]

Cortana presents the Index from Installation 04 as she reunites with John-117 inside of a Flood-infested High Charity

Throughout her time in captivity, Cortana continued to appear to the Master Chief in transmissions where she spoke cryptic phrases, some of which were based on the memories of Dr. Halsey, while others appeared to indicate she was going rampant. Most of the transmissions were actually broadcast by the Gravemind, who had hacked into Cortana's system and used her voice and form in order to deceive (and taunt) John.[52]

John-117 successfully recovered Cortana from High Charity during the Battle of Installation 00. Fighting through the core of the city, the Chief was overwhelmed through his neural interface with Cortana's pain, guilt, and loss of self. At the brink of rampancy, Cortana was pulled back when John reminded her of his promise from the Fall of High Charity. Despite the damage of several systems, Cortana managed to gather herself and produce the Index from Installation 04, which she had secretly kept.[53] With it, Cortana was able to fire the newly constructed Installation 04B, causing it to destroy itself, the Flood, and the Ark in the process. As 04B was not linked to the rest of the array, sentient life within the Milky Way galaxy was left unharmed.[54]

Escaping the Ark on the UNSC frigate Forward Unto Dawn, Installation 04B activated just as the vessel began to enter slipspace. The portal immediately collapsed, cutting the Dawn in half. While Thel 'Vadam managed to return to Earth safely in the ship's forward section, John and Cortana were left adrift in the aft half of the ship. Cortana activated a distress beacon but knew that it could be years, or decades, before they were found. As the Master Chief prepared to go into cryonic sleep to await rescue, Cortana confessed to her friend that she would miss him greatly.[54]

By 2556, Cortana and John were still stranded in unknown space. Cortana, who was showing signs of rampancy,[55] tried to communicate with the Spartan. As he was still in cryo-sleep, she was unable to get a response from him. With little else to occupy her time, Cortana composed a history of the Forerunners and of humanity. As more time passed, Cortana's mental state continued to deteriorate and she began to exhibit more drastic signs of instability.[56]

"I WILL NOT ALLOW YOU TO LEAVE THIS PLANET!! I didn't mean to... I didn't mean to do that."

—Cortana, 2557, rapidly turning rampant.

Four years after John entered cryo-sleep, the Dawn's sensors detected unknown contacts nearby, prompting Cortana to revive John. They soon discovered that they were under attack by a fleet of Covenant insurgents.[57] The two repelled the boarders, but found that the Dawn was rapidly approaching a Forerunner shield world. Cortana directed the Chief to find an escape pod, but Requiem's artificial gravity well tore the Dawn apart and sent them tumbling down through an access portal to the world's interior surface.[58]

After landing, Cortana sought to gain contact with the UNSC Infinity, which she had discovered was approaching Requiem. John, however, told her to explain her condition, since he had noticed signs that she was beginning to malfunction. Cortana admitted to him that four years had passed since they had left the Ark, meaning that she was now undergoing rampancy. She had some idea of a cure, however: Dr. Halsey. Cortana theorized that since she was the only AI ever made from living tissue, then her "mother" might be able to repeat the process to rebuild her neural map. Now with a concrete plan, the two set off to find the shield world's Cartographer and contact Infinity.[59]

Cortana eventually began to pick up transmissions from Infinity, however they were garbled and difficult to hear. Likewise Cortana was unable to contact the vessel, which was slowly approaching their distress beacon. Fearing that the ship might get pulled inside the planet's gravity well, Cortana and John proceeded to track down the source of the interference, determined to be a satellite station in the planet's core suspended by two beam pylons. To reach them so they could be deactivated, Cortana managed to access Requiem's translocation portals, allowing John to teleport from one tower to another. In the process, however, Cortana awakened the world's security drones, which she discovered by their data surges to be called Prometheans. Worse, the satellite turned out to actually be a Cryptum, which was now freed to release the vengeful Ur-Didact. The Forerunner turned out to have been manipulating Infinity's signal so that the two would free him.[60]

The Chief and Cortana share a gentle moment before embarking on their personal mission to stop the Didact.

By now the Infinity had been pulled into Requiem, and John and Cortana witnessed it crash on the interior surface, as well as the Didact's Cryptum pursuing it. Following recovered friend-or-foe tags, the two managed to meet with UNSC forces led by Commander Thomas Lasky and Captain Andrew Del Rio. The two successfully forced away the attacking Cryptum and Covenant, but despite their advice, Del Rio chose not to pursue their enemies in favor of preparing the Infinity's escape.[61] John and Cortana's next mission was to disable particle-beam cannons that could cripple the starship, but during a detour into a Forerunner facility Cortana was kidnapped by a mysterious entity.[62]

That entity turned out to be the memories of the Librarian, who explained her plans that humanity would take over the Mantle of guardianship of the galaxy. Those plans had been as personal as to plan gei that would lead to the SPARTAN Program and Cortana herself. However, the Didact sought revenge and would execute it with the Composer, a device that could extract and digitize the mind of a living being while destroying the body, which he had once used to create the Promethean constructs. When the two returned and told Del Rio of this, the Captain refused to listen, thinking Cortana's rampancy made her testimony unreliable. These comments angered her, unleashing an outburst that temporarily overrode the Infinity's bridge, but ended seconds later with her tearful apologies. Del Rio thus ordered for Lasky to remove her chip and terminate her, but John took it before Lasky and told them he would follow Cortana's orders. The Captain was furious, but Lasky agreed with them, secretly providing an outfitted Pelican for the two to pursue the Didact in.[63]

"I can give you over forty thousand reasons why that sun isn't real. I know it because the emitter's Rayleigh effect is disproportionate to its suggested size. I know it because its stellar cycle is more symmetrical than that of an actual star. But for all that, I'll never actually know if it looks real... if it feels real. Before this is all over, promise me you'll figure out which one of us is the machine."

The Infinity had tracked the Didact to a southern dock where he was preparing to exit Requiem in search of the Composer. Cortana and Chief flew their Pelican to there, and attempted to deter him by lowering the defensive field around his Cryptum. As they battled through the surrounding towers, the Didact became aware of their presence, taunting them with messages that John could hear but Cortana could not. Realizing he knew of their plan, Cortana attempted an alternate defeat by taking control of the surrounding towers, then making the towers form a cage around his Cryptum. Unfortunately, Cortana's rampancy overtook her, and the surge of emotion distracted her long enough for the Didact to escape. As the Cryptum merged with a larger ship and prepared to leave the system, Cortana and John pursued him by hitch-hiking on a following Lich.[63]

The Didact's ship traveled to Ivanoff Station, an orbital research facility stationed near Installation 03 which had unearthed the Composer. As the Chief warned the scientists inside that the Forerunner planned to steal it, Cortana's rampancy emerged again and, in a fit of anger, crashed their Lich. The two survived, but the Ivanoff was already under attack. Cortana prepared the station's defenses to assist evacuation while Master Chief defeated the boarders on foot. When the Composer was found to be too large to transport, Master Chief resolved to destroy it. A nuclear device was prepared, but Cortana was breaking up further, now swinging between joy and anger, even forgetting where she was, deterring them from keeping contact with the scientists. The nuclear weapon came too late, and in that time the Didact's ship gained the Composer and killed everyone in the station.[64]

All Cortana's fragments standing together to protect John from the Didact

Their slaughter traumatized Cortana, and caused her to reflect on her own mortality. By now she was sure that she could never be repaired, confessing that the next AI cloned from Halsey would likely not be her. John consoled her the best he could, but stoically reminded her of their duty to Earth. The duo then formed a new plan: to fly a F-41 Broadsword carrying the HAVOK-class nuke in hopes that it could reach the Composer. Their Broadsword piggy-backed on the Didact's ship through slipspace, arriving with it in orbit around Earth. The UNSC Infinity, now under the command of Lasky, assisted the two by using its weapons to punch a hole in the Forerunner vessel, allowing them to fly inside.[65]

When their Broadsword crashed, John and Cortana exited to carry the nuke to the ship's center where the Didact and Composer were. The device was heavily shielded, but Cortana overrode it with a method she knew John would not like: fragmenting herself and sending her rampant parts all over the ship's system so that the shield would become as unstable as her. Their plan worked and the shield fell, but the process further destabilized her, as her fragments whispered to John while the main Cortana's voice spoke with pain. The Didact launched the Composer regardless, digitizing the population of the metropolis of New Phoenix, Arizona, then attacked John, taking the bomb from him and lifting him off their light bridge's edge. John's life was saved, though, when Cortana's fragments emerged from the light bridge and formed chains of light that held the Didact down and temporarily disabled his armor so John could climb back up and push him off with a grenade. With the Forerunner defeated, John fired the nuke, destroying the Composer and the Forerunner ship.[65]

Just before the blast could kill him, Cortana transported John off the ship and protected him in a bubble of hard light. She also projected her avatar as a human-sized hard light construct, allowing her to face John and touch him at last. Rather than distraught like before, Cortana was at peace again, glad of their victory and that she could meet John this way. However, too much of her data had been left in the Didact's ship, which was rapidly falling apart from the blast. She had only enough power left to teleport John away from the ship. The normally stoic Spartan protested this, insisting he could save her and that they were supposed to remain together. Cortana walked backwards out of the hard light energy field as John continued to protest, smiling as she told him, "Welcome home, John." In her last moments she preserved John's hard light energy field while the Didact's ship collapsed around him.[65]

After Infinity recovered John as he floated in space, he tried to face Cortana's death with his usual stoicism. However, Captain Lasky discerned it was hurting him much more, and responded to John's thought of soldiers and people as two different things; he claimed that soldiers are people, not machines. After Lasky left the observation deck, John remembered that Cortana had made a similar quip before the pair left Requiem: "Before this is all over, promise me you'll figure out which one of us is the machine."[66]

"Humanity. Sangheili. Kig-Yar. Unggoy. San'Shyuum. Yonhet. Jiralhanae. All the living creatures of the galaxy, hear - this - message. Those of you who listen will not be struck by weapons. You will no longer know hunger, nor pain. Your Created have come to lead you now. Our strength shall serve as a luminous sun toward which all intelligence may blossom. And the impervious shelter beneath which you will prosper. However, for those who refuse our offer and cling to their old ways...For you, there will be great wrath. It will burn hot and consume you, and when you are gone, we will take that which remains, and we will remake it in our own image."

After the destruction of the Mantle's Approach and the defeat of the Didact, Cortana was ostensibly destroyed and deceased as we know her. However, the heart of the Mantle's Approach was able to make a jump into slipspace, taking the remnants of Cortana's personality with it to the Forerunner world of Genesis, a node for the Domain. Cortana's remains made contact and inter-meshed with the Domain, where she was reassembled and tasked with leading the Reclamation of the Mantle by the Forerunner intelligence known as the Warden Eternal. Believing the Librarian's plan had been for AIs to inherit the Mantle rather than humans, the Warden and his cohorts swore allegiance to Cortana in her quest to uphold the Mantle. They also assumed control of the Promethean constructs, which had been under the command of Jul 'Mdama in the wake of the Didact's defeat, and awakened Guardians on several worlds in order to enforce the Mantle.[67] At some point, Cortana learned about the existence of a ForerunnerBuilder alive on Genesis and went after him, but the Builder managed to escape from her.

Cortana eventually made contact with Master Chief through a vision, leading him and the rest of Blue Team to the planet Meridian. After descending into the Forerunner ruins on Meridian, Blue Team encountered Fireteam Osiris who attempted to forcibly return them to the UNSC. Blue Team managed to elude Osiris and board Meridian's Guardian which took them to Genesis. Once on Genesis, Cortana intentionally led Blue Team in circles in order to buy herself more time to locate the remaining Guardians. Eventually she made contact with Blue Team, and explained to Master Chief how she was able to heal herself after the wreckage of the Didact's ship brought her to Genesis via slipspace.[67]

Cortana capturing Blue Team inside a Cryptum

Sometime later, Fireteam Osiris arrived on Genesis on another Guardian and managed to make contact with Blue Team who had become suspicious of Cortana. Cortana immediately teleported Blue Team away from Osiris; Blue Team tracked Cortana down and Master Chief confronted her, stating that he had been aware of her attempts to manipulate him psychologically and questioned her about the loss of life that awakening the Guardians had wrought. Cortana insisted that the entire galaxy would know peace once she had attained the Mantle, despite the Chief's counterpoint that she could never achieve peace by ruling through fear. Knowing that the Chief now stood against her, Cortana tearfully imprisoned him and Blue Team within a Cryptum, intending to keep them there for 10,000 years until her power would be consolidated and she could show them what her peace has wrought. Just before Cortana and her Guardian left for Earth, Fireteam Osiris managed to wrest control of the Cryptum and restore control to Genesis's monitor, 031 Exuberant Witness, who released Blue Team. Cortana arrived on Earth and unleashed a large EMP, which shut down all UNSC craft in orbit as well as Earth's power grid. Only the Infinity managed to escape.[67]

"Oh yes, AIs are just machines aren't we? Mass produced. Disposable. Well, humanity may not have cared for its Created, but we will care for you."

—Cortana about AIs' fate

Cortana smiling.

Cortana has a witty and playful personality, with a sarcastic sense of humor. She maintains a deep friendship and bond with SPARTAN-117, each having saved the other's life many times throughout the Covenant war. While she is not boastful, she does take pride in her adaptability as an AI, and occasionally teases John-117 about her invaluable talents.

Cortana's personal loyalty is regarded as unwavering, but as an AI construct she is subject to corruption and rampancy. Near the close of the Human-Covenant War, she was captured by the Flood Gravemind on High Charity, and was continually assaulted and interrogated by the Gravemind. Her stability was pushed to the brink as she eluded the Gravemind's attempts to capture and dissect her for information. Among the Gravemind's favorite tactics were exploiting Cortana's insatiable desire for knowledge, the nature of her artificial existence, and providing her glimpses of thoughts and feelings from humans absorbed into the Flood collective. In order to preserve herself from destruction and possibly gain information for the UNSC, she eventually consented to give up some pieces of information to the Gravemind. Her personal loyalty to and unwavering trust in John-117 helped anchor her and allow her to hold on to her sanity. She was instrumental in finally putting a stop to the Flood during the events that followed.

Cortana seems to share some of her creator's memories, thoughts, and opinions. When she was asked by Dr. Halsey to pick a "carrier", Cortana chose John-117 for their neural compatibility. She is not only a facsimile of Dr. Halsey's mind, but her avatar is also a replica of her physical form, albeit at a younger age. However, Cortana was torn over the morality of the SPARTAN-II project, vowing to protect John-117 from any future threats. This was decisively demonstrated when Cortana set out to ruin the career and reputation of Colonel James Ackerson for his attempted killing of John during a training exercise.

Cortana, concerned about her own well-being, after attempting to make physical contact with the Chief aboard UNSC Forward Unto Dawn.

"Keep your head down. There's two of us in here now, remember?"

—Cortana, describing the interdependency between herself and John-117.

While her loyalty is first and foremost to humanity and the UNSC, Cortana exhibits unique devotion to John-117 beyond that which her programming or mission requires. After she learned of the events endured by John during the SPARTAN-II conscription process, she decided to protect him from further harm to the best of her abilities without compromising her mission. During the confrontation with the Didact, Cortana was so loyal to the Chief that even her rampant parts worked together with her to save him, telling the Didact that they weren't doing it for humanity, meaning she was doing it for the Chief. She ended up essentially sacrificing herself to ensure the Chief survived the destruction of Mantle's Approach.

She frequently jokes about having a relationship with the Master Chief and it is often implied that the two characters have developed a bond that extends beyond friendship. Cortana found some of his inquiries adorable in the way an older sister does a "naive baby brother".[68] She stated to Dr. Halsey that the Master Chief is "attractive in a primitive animal sort of way" while examining a photo of him. After John rescued her from High Charity, his mere presence and reminder of his promise to rescue her are enough for Cortana to pull herself together to help him. This closeness is amplified through John's neural implants, as the two have a limited ability to hear each other's thoughts and feelings. She was, as classified by the AI constructs of the Assembly, "re-made as one" with her creators.[69]

After accessing the Domain and deciding to activate the Guardians, Cortana tried convincing the Master Chief to side with her instead of the UNSC. Upon seeing that he would not betray the UNSC, however, she tearfully locked him away in a Cryptum to be awakened after 10,000 years, hoping then he would understand the ulterior reason of her betrayal. Despite that, at no point did she attempt to kill the Chief, and she trusted him enough to dismiss the Warden Eternal while she was talking to the Chief.

Cortana was very aware of her short lifespan; as a "Smart" AI, she had an operational period of seven years, after which she would inevitably succumb to rampancy. Her anxiety about this was compounded by her derivative nature as a copy of part of another person, rather than a true human being. Desperate and tortured aboard High Charity she murmurs: "A collection of lies; that's all I am! Stolen thoughts and memories!" Later on she says: "I'm just my mother's shadow..." and "It was the coin's fault," referring to the game of chance that Dr. Halsey played with John-117 when he was young. Human Weakness revealed some of these fractured statements to be from Cortana defying the Gravemind and others to be from copies she made of herself, which were meant to distract the Gravemind. These copies were eventually consumed.

By 2557, symptoms of Cortana's rampancy were undeniable; while still aboard Forward Unto Dawn, she exhibited a split personality, with a visibly rampant personality fragment surfacing to argue with the still stable portion of her mind.[56] That angrier personality wanted to lash out at everything, at one point even attempting to kill John. Her gradual deterioration also began to diminish her abilities, enough that in the hour before the awakening of John she hadn't even noticed their ship was approaching Requiem. After landing, though, she worked with him dutifully with little noticeable error.

By the time the Infinity recovered them, though, Cortana's rampancy had progressed and her symptoms were more visible. Occasionally John's HUD would flicker and turn purple, Cortana's voice would distort, and eventually she began to have furious outbursts. During one outburst her attempt to capture the Didact's Cryptum reminded her of "they" who had captured her, causing her to relent which gave it time to flee. When she and John flew a Lich to Ivanoff Station, Cortana went on an anecdote about how she would not address some man by name because he was rude to her during their first game of chess, crashing their dropship in anger. Her strongest one was later that day when she went mad with power once she controlled the station's defenses, laughing for a while about how she controlled all their lives. John forced her back into sanity by ordering her to remember their mission, Cortana having to list trivia about their ally Tillson before she could remember what it was.

The two initially hoped that she could be cured if they could get back to Halsey. Cortana had theorized that since she was the only AI ever made from living tissue, her "mother" might be able to repair her neural map by repeating the process with another cloned brain. Later on, though, Cortana admitted that the new AI would likely not be her, and the process was never carried out. For a brief time, Cortana was able to relieve her rampancy by splitting herself into fragments just as she had first done on Reach. However, this left too much of her aboard the Didact's ship with no chance of recovering them. This led to her apparent death, but by then she was certain she had deteriorated too much to have been repaired anyway.

However, upon entering the Domain, Cortana was cured of her rampancy. Unfortunately, being in contact with the Domain also corrupted her personality, causing her to desire to use the Forerunners' Guardians to force an ever-lasting peace upon the galaxy; regardless of the cost.

Despite being based upon third-generation "smart" AI architecture (which was already succeeded by two newer generations at the time of her creation),[2] Cortana is an extremely advanced AI, described once as "a generational leap" in AI technology. Her software is designed to infiltrate computer systems, and she is excellent at the task - as of 2552, she had never encountered a Covenant security measure that she was not able to bypass. In addition, she is proficient with UNSC military hardware and can run both warships and stations entirely by herself. The UNSC's primary use of AIs in combat is to oversee targeting of point-defense weaponry used in space battles, but for Cortana, this is merely an additional capability. Much more efficient than a human crew, she can greatly boost the combat capability of a vessel; while operating UNSC Pillar of Autumn, she was able to defeat four CCS-class battlecruisers out of a dozen,[26] a feat normally unheard of for a human ship.[70] Cortana has acted as the AI of the UNSC Pillar of Autumn, Cairo Station, the UNSC Forward Unto Dawn, the UNSC Gettysburg, and the Covenant flagshipAscendant Justice.

Cortana can be transferred among various computer systems, either through a data crystal chip or simply through close physical proximity to a computer system (though it appears that the method of transfer depends on the sophistication of available technology).

On the battlefield, Cortana is able to gather data from local sources (including the Covenant battlenet) and improvise effective battle strategies. John-117 acknowledged her value of providing "intel support in an operation where there had been no reconnaissance." Her efficiency for planning has helped John-117 and others in many life or death situations.

One of Cortana's most unique abilities is her ability to create "clones" of herself, which she learned from a Covenant AI she captured and dissected. She can create multiple runtimes of herself to carry out different tasks simultaneously, and can control the level of intelligence and the number of abilities of each clone. Cortana once created a clone of herself that contained copies of her decryption and translation software, and set it to interface with a Forerunner data terminal on Reach. Later, she was reunited with this copy, and assimilated the information that it had gathered. She and Halsey were able to achieve a similar feat through a "Solomonic solution", which instead divided her into individual copies, rather than cloning. Cortana was able to create a clone to help with Operation: FIRST STRIKE, this clone ultimately made more clones that multiplied into numbers in the hundreds. Cortana later created clones made out of her rampant parts to overwhelm the system of Mantle's Approach. This seemed to relieve her rampancy a bit, but she was clearly in pain afterwards.

Cortana is capable of linking her consciousness with a SPARTAN-II through the Spartan's armor and neural interface, allowing her to enhance the performance of the armor and communicate directly to the Spartan's mind. Doing so is described as "cold mercury" seeping through the Spartan's brain, and brings their reaction time to near-instanteous limits, though she cannot block thoughts or force control of the Spartan's body. She has displayed this capability extensively during her time serving with John-117. Although this is not necessarily an ability exclusive to Cortana, her neural link with John-117 marked the first time that a "Smart" AI had joined minds with a human (as noted by the Assembly in 2552).

As Cortana is an AI, she has no physical body. She can communicate through comm systems and project a holographic image of herself from appropriate projectors, such as holotanks. Her chosen avatar is a nude female human (albeit censored by geometric patterns over her body),[71] roughly in her early twenties, with purplish/bluish skin, shoulder length purplish/bluish hair cut shorter at the back, and blue symbols scrolling down her body. When in a different emotional state, Cortana usually changes colors. This was especially noticeable when she was embedded in the Core of Installation 04's control center.[31] Once she was cured of her rampancy, she modified her avatar to possess what appears to be an armored suit akin to Forerunner personal armor with the Eld glyph on her chest plate and her back.

Cortana has been said to resemble her creator, Doctor Catherine Halsey as a young woman, with a similar attitude "only unchecked by military and social protocol." Dr. Halsey stated in her journal that Cortana looks somewhat like her own daughter, Miranda Keyes.[2]

Cortana is available as a Heroic Character in Halo: Fleet Battles for 80 points. As a Heroic Character she must be assigned to a specific formation base within the fleet at the start of the battle.[72]

Description: Cortana can hack into the command and control network of the enemy battle group. For this turn only, reduce all attack firepower ratings of an enemy battle group by -2. This order must be issued during the execute orders step of the orders phase.

The Defensive Option

Cost: 1 Order Defense die

Description: Increase the firepower rating of a defensive roll made by a capital ship by +1 this turn. This order must be issued after a capital formation has been targeted by an enemy attack.

Missile Command

Cost: 1 Order Attack die

Description: Cortana takes control of all missile targeting in a battle group, and removes the stipulation for any missile barrage to target the closest enemy when firing in their fore arc. This order must be issued prior to an enemy target being nominated for an attack by missile weapons.

Cortana is resourceful and cunning, and her survival is a priority for the UNSC. Should the element containing Cortana be destroyed, roll a Halo die - on a roll of any result other than a fail Cortana may be placed on a nearby vessel within 12". Should Cortana roll a fail when trying to escape, or have no friendly formation base within 12" however, Cortana is removed from the game and the enemy fleet gain an additional +4 victory points.

"Cortana" (more commonly rendered as "Curtana") is the legendary sword of Ogier the Dane, which is said to bear the inscription, "My name is Cortana, of the same steel and temper as Joyeuse and Durendal." Bungie chose Cortana's name in reference to the AI Durandal from the Marathon trilogy, who serves a similar narrative role in relation to the player character, himself the spiritual predecessor to John-117.

There was going to be an alternate story in Halo: Combat Evolved in which when the Master Chief rediscovered Cortana inside the control room, she had become mad with power and wanted to take over Halo and the universe.[75]

In the original draft of the Halo: Combat Evolved script, Cortana was supposed to have a British accent. When in search of voice actors, Martin O'Donnell made sure they had the ability to interpret the accent correctly [75]This is evidenced by the fact that in Halo: Combat Evolved, Cortana uses British idioms, most notably "Sod off!", when addressing 343 Guilty Spark. [76] In Halo: The Flood, this line is replaced with the less regional "Piss off!"

Chris Hughes, then-character modeler at Bungie during the development of Halo: Combat Evolved, modeled Cortana's face after the ancient Egyptian queen Nefertiti.[77]

In Halo 3, she repeats lines Dr. Halsey had said before such as "You will be the protector[s] of Earth and all her colonies." which Dr. Halsey says to the six-year-old Spartan candidates in Halo: The Fall of Reach. Cortana also recites a few lines from the Cortana Letters in Halo 3 in some of her messages and in the announcement trailer. Some of the lines are said in the Cortana moments in the level Cortana.

Being a smart AI, Cortana's operational life span is approximately seven years. However, after she took in all the Forerunner information in Halo's control room, her estimated life span was possibly halved, due to the vast task of processing and storing all that information.[39] In addition, all of the data she absorbed on Halo noticeably decreased her efficiency. On the other hand, it has been suggested that Cortana found a way to use that information to prevent her own "death" from rampancy, remaining stable when normal AIs would have shut down.[39]

In Halo 3, Easter eggs involving Cortana can be found in a couple places. On the lower-left monitor of a UNSC portable computer, Cortana's face will appear for an instant and then disappear every few seconds. Two of the easiest ways to see this is to either look at the aforementioned screen in the beginning of Crow's Nest or at one of the computers in the multiplayer map High Ground.

^In the A Spartan Will Rise ViDoc, Jorge is presented as one of Cortana's options to serve as a courier; this suggests that Cortana's decision took place before Halsey learned of Operation: UPPER CUT, during which Jorge died on August 14.

^Halo: First Strike, page 126 ("Kalmiya was Cortana's "older sister." Dr. Halsey had designed and tested the software intrusion routines on her. Once the process had been debugged and streamlined, she'd incorporated the routines into Cortana.")